The present invention concerns a steel for machine structural use having good machinability and good chip-breakability in cutting by cemented carbide tools, as well as a method of producing the steel. The invention also concerns a steel for machine structural use exhibiting high fatigue strength and bend-straightenability in addition to the good machinability and chip-breakability.
In the present invention the term xe2x80x9cduplex inclusionxe2x80x9d means an inclusion of the structure in which a core mainly consisting of oxide inclusion is surrounded by another inclusion consisting mainly of sulfides. The terms xe2x80x9ctool life ratioxe2x80x9d and xe2x80x9clife ratioxe2x80x9d mean a ratio of tool life of the free-cutting steel according to the invention to tool life of the conventional sulfur free-cutting steel containing the same S-contents in turning with a cemented carbide tool. xe2x80x9cFinely dispersedxe2x80x9d MnS inclusion particles are the particles finer than those in the conventional MnS inclusions particles contained in the conventional steel and existing in the form of uniform distribution in the steel matrix without coagulation or concentration.
Research and development on steel for machine structural use having high machinability have been made for many years, and the applicant has made many proposals. In recent years Japanese patent disclosure No. 10-287953 bearing the title of xe2x80x9cSteel for machine structural use having good mechanical properties and drilling machinabilityxe2x80x9d is mentioned as one of the representative technologies. The free-cutting steel of this invention is characterized by calcium-manganese sulfide inclusion containing 1% or more of Ca in a spindle shape with an aspect ratio (length/width) up to 5, which envelopes a core of calcium aluminate containing 8-62% of CaO. Though the steel exhibited excellent machinability, dispersion of the machinability has been sometimes experienced. This was considered to be due to variety of types of the above-mentioned calcium-manganese sulfide inclusion.
The applicant disclosed in Japanese patent disclosure No. 2000-34534 xe2x80x9cSteel for machine structural use having good machinability in turningxe2x80x9d that, with classification of Ca-containing sulfide inclusions into three groups by Ca-contents observed as the area percentages in microscopic field, A: Ca-content more than 40%, B: Ca-content 0.3-40%, and C: Ca-content less than 0.3%, a steel satisfying the conditions, A/(A+B+C)xe2x89xa60.3 and B/(A+B+C)xe2x89xa70.1, exhibits very prolonged tool life in turning.
Further research by the applicant succeeded, as disclosed in Japanese patent disclosure No. 2000-219936 xe2x80x9cFree-cutting steelxe2x80x9d, in decreasing the dispersion of the machinability by clarifying necessary number of inclusion particles in the steel. The steel of this invention is characterized in that it contains five or more particles per 3.3 mm2 of equivalent diameter 5 xcexcm or more of sulfide inclusion containing 0.1-1.0% of Ca. There was, however, still some room for improving the dispersion of the machinability.
Then the applicant developed a steel for machine structural use having improved dispersion of the machinability, with so high machinability in cutting by a cemented carbide tools that fivefold or more of tool life ratio is achieved, and proposed it (Japanese patent application No. 2001-174606 xe2x80x9cFree-cutting steel having good machinability in cutting with a cemented carbidexe2x80x9d). The free-cutting steel is characterized by the state of the inclusions. The characteristic feature of the steel is the above-mentioned xe2x80x9cduplex inclusionxe2x80x9d, i.e., the inclusion of the structure in which xe2x80x9csulfide inclusion particles containing Ca of 1.0 wt. % or more and neighboring to oxide inclusion particles containing CaO of 8-62 wt. %xe2x80x9d exists at least a certain amount, specifically, xe2x80x9cthat the area occupied by the sulfide inclusion is 2.0xc3x9710xe2x88x924 mm2 per 3.5 mm2 or more in microscopic fieldxe2x80x9d.
In the patent application there has been disclosed that in a method for producing the free-cutting steel containing the above mentioned duplex inclusions it is essential to carry out the operation satisfying the conditions below at preparation of the steel:
[S]/[O]: 8-40,
[Ca]xc3x97[S]: 1xc3x9710xe2x88x925-1xc3x9710xe2x88x923 
[Ca]/[S]: 0.01-20, and
[Al]: 0.001-0.020%.
Based on the recent results of research the applicants developed a free-cutting steel having not only a long tool life but also good chip-breakability, and therefore, suitable for being processed with automated machining. This free-cutting steel is already proposed (Japanese patent application No. 2001-362733). The free-cutting steel consists essentially of, as the basic alloy composition, by wt. %, C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.020%, Ca: 0.0005-0.02%, O: 0.0005-0.01% and N: 0.001-0.04%, and further, one or both of Ti: 0.002-0.010% and Zr: 0.002-0.025%, the balance being Fe and inevitable impurities, and is characterized in that the area occupied by the sulfide inclusion particles containing Ca of 1.0 wt. % or more and neighboring to oxide inclusion particles containing CaO of 0.2-62 wt. % is 2.0xc3x9710xe2x88x924 mm2 per 3.5 mm2 or more of microscopic field and that the above MnS inclusion particles are finely dispersed in the steel.
Novel features of this invention in comparison with the previous invention are, in one hand, extended lower limit of CaO-content of the oxide inclusion particles which form the duplex inclusion, and on the other hand, a more important difference that xe2x80x9cthe MnS inclusions are finely dispersedxe2x80x9d. The latter feature brings about the improved chip-breakability, and as the result, realizes suitable balance of the tool life and the chip-breakability. The former feature, fine dispersion of the MnS inclusions can be given by addition of a certain amount or amounts of Ti and/or Zr to form fine Ti-oxide, Zr-oxide or (Ti+Zr)-oxide to have MnS precipitated on the oxide nuclei. These oxides may include manganese oxide, and thus, in that case, they may be TiO2xe2x80x94MnO2, ZrO2xe2x80x94MnO2 or TiO2xe2x80x94ZrO2xe2x80x94MnO2.
The free-cutting steel covers various kinds of steel classified in the steel for machine structural use. It has been found that, in the process of establishing concrete alloy compositions in the applicable fields, the invention is useful even in the relatively high S-content range. In other words, it has been revealed that, of the operation conditions mentioned above, the upper limit of [S]/[O]: 8-40 can be increased to 80 or so. On the other hand, after wide-ranged experiments, some dispersion in the balance of the tool life and the chip-breakability was still observed.
The object of the present invention is to add further improvements to the free-cutting steel for machine structural use having improved dispersion of the machinability by utilizing the above described duplex inclusion to enable such improvement in machinability as fivefold or more tool life and good chip-breakability, and thus, to provide a steel in which better chip-breakability can be always obtained and is suitable for machining, particularly, turning. To provide a free-cutting steel for machine structural use having, in addition to the secured balance of the machinability and the chip-breakability, good fatigue strength and good bend-staightenability is also included in the object of the present invention.
The free-cutting steel for machine structural use according to the present invention achieving the above-mentioned object, or the steel having good machinability as well as good chip-breakability consists essentially of, as the basic alloy composition, by wt. %, C: 0.05-0.8%, Si: 0.01-2.0%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.020%, Ca: 0.0005-0.02%, O: 0.0005-0.01% and N: 0.001-0.04%, and further, one or both of Ti: 0.002-0.010% and Zr: 0.002-0.025%, the balance being Fe and inevitable impurities, and is characterized in that the area occupied by the sulfide inclusion particles containing Ca of 1-45 wt. % and neighboring to oxide inclusion particles containing CaO of 0.2-62 wt. % and having melting point of 1500-1750xc2x0 C. is 2.0xc3x9710xe2x88x924 mm2 per 3.5 mm2 or more in microscopic field, and that the sulfide inclusion particles other than the above defined sulfide inclusion particles are finely dispersed as MnS in the steel.